1. Field of the Invention
The invention relates generally to energy (shock) absorbing devices, and more particularly to a friction dampener adapted to railway vehicle motion control.
2. Description of the Prior Art
Various types of shock absorption and dampening devices have been employed for many years in a variety of different applications. The most prevalent of such devices is probably the shock absorber, which is frequently used in a variety of different kinds of vehicles. The most common type of shock absorber is probably the hydraulic shock absorber, which utilizes a fluid filled cylinder and a plunger mechanism. Typically, the cylinder housing and the plunger are separately connectable to external elements which are expected to undergo relative displacements. However, there can be many applications where an energy absorber is called for, but where the hydraulic shock absorber is not necessarily desirable for many possible reasons. For example, modern hydraulic shock absorbers are relatively complex, expensive, and can be much heavier than friction type dampening devices. Furthermore, because of the fluid medium utilized in hydraulic shock absorbers, the orientation of such shock absorbers can have an effect on performance. For example, hydraulic shock absorbers are generally more efficient when mounted in a relatively vertical orientation. Yet, there can be many applications where it is desirable to mount a shock absorber in a horizontal orientation. In particular, one such application is where a shock absorber, or dampener, is utilized as a yaw dampener on a railway vehicle truck assembly to control hunting.
Friction type shock absorbing devices, such as a friction dampener, can be simpler in design, less expensive, lighter weight, and unaffected by the orientation in which it is mounted. Various types of friction dampeners have been employed in the past to provide shock absorption or cushioning features where the use of hydraulic shock absorbers was undesirable. Prior friction dampeners can be one or two way energy absorbers and commonly comprise a first member, such as a cylindrical housing, a second member such as a shaft or rod, which is coaxially disposed within the housing, and a friction pad assembly carried by the shaft in sliding engagement with the inside surface of the housing. Both the housing and the shaft or rod are provided with a connecting member for attachment to external elements which are expected to undergo relative displacement. Such a friction pad assembly generally includes a friction element and some type of wedge member to initially set (or reset) the pressure between the friction element and the inside surface of the housing. Two examples of such friction dampeners are disclosed in U.S. Pat. No. 3,866, 67,724 and U.S. Pat. No. 3,796,288, both to Holnick. The friction dampener disclosed in both of the aforementioned patents provide a manual adjustment means for moving a wedge shaped member between, or out from between, the friction element in order to increase or decrease the force applied to the friction element against the inside surface of the housing.
Other friction dampeners have provided a leaf spring to bias the friction element against the housing. One such device is disclosed in U.S. Pat. No. 3,121,218 to Hallinan. Such a device can have a leaf spring that is bimetallic and manually adjustable to either urge the friction element against the housing to increase the frictional engagement, or reduce such pressure to decrease friction.
Still other friction dampeners have provided for a small servo motor to automatically adjust the wedge member to increase or decrease the pressure between the friction element and the inside surface of the housing. One such device disclosed in U.S. Pat. No. 5,080,204 to Bower et al. discloses a friction dampener for the drum of a washing machine unit. In Bower, a small servo motor is provided inside the dampener to operate a piston which moves the wedge member in order to decrease the pressure between the friction element and the inside surface of the housing. The servo motor is a thermoactuator element that is responsive to a rotational speed sensor which triggers the servo motor when a predetermined rotational speed has been exceeded.
However, such friction dampener devices can be associated with certain disadvantages resulting from the nature of the friction element when such devices are employed in some heavy load applications. An example of such an application is the use of a friction dampener on a railway vehicle truck assembly to control hunting, as referred to previously, wherein the truck assemblies can be carrying hundreds of tons of materials. During operation of a friction dampener in such an application, the extremely large forces which the dampener must control can result in very high temperatures being generated by the frictional interaction between the housing and the friction element. Because the friction element typically expands in response to an increase in temperature, this can causes a corresponding increase in the peripheral pressure on the housing. As might be expected, this increase in peripheral pressure normally results in causing the dampener to become increasingly stiffer. Compounding the situation further, the friction coefficient of the friction element is typically sensitive to temperature. Thus, as the temperature increases the friction coefficient of the friction element usually also increases. This is analogous to the situation where race car drivers spin the tires on the car to get the tire temperature up so they stick to the track better and resist slipping. A rubber/elastomeric compound experiencing this condition is commonly referred to as being xe2x80x9ctacky.xe2x80x9d Thus, like the race car tires, the friction element can resist sliding on the inside surface of the housing as the temperature increases. The result can be that during operation of the friction dampener the pressure becomes so great and the friction element becomes so tacky that essentially no relative movement can occur between the friction element and the housing in the normal operating range. Basically the friction dampener can xe2x80x9clock up,xe2x80x9d at which point the dampener begins to behave like a fixed rod. This very undesirable condition can result in damage to the friction dampener or the externally connected elements which are expected to be able to move relative to each other. Furthermore, since the yaw forces are not being dampened, hunting of the truck assembly can get out of control.
Therefore, friction dampeners which do not provide some means for controlling the peripheral pressure between the friction element and the housing can be unacceptable in certain applications where the friction dampener must control heavy loads and undergo large variations in temperature and pressures. Accordingly, there is a need for a friction dampener which can generally maintain the peripheral pressure between the friction element and the housing within a preferred range of acceptable operating pressures.
In accordance with the present invention there is provided a friction dampener which can generally maintain the peripheral pressure between the friction element and the housing within a certain preferred range of acceptable operating pressures. The friction dampener can be mounted in any orientation without loss of performance, can be lighter in weight and less expensive to manufacture than a typical hydraulic shock absorber.
Such a friction dampener can include a housing having one end slidably surrounding one end of a shaft movable relative thereto. Opposing ends of the housing and shaft can each having a connecting eye member for connection to separate independently movable elements. One or more friction elements can be carried by the shaft which frictionally engage inner surfaces of the housing. The friction elements can have inner and outer portions having different material properties. The housing can be adapted to generally maintain the pressure between the friction elements and the housing within a range of desirable pressures.
In one embodiment, the friction dampener can have a generally cylindrical housing slidably surrounding a tubular shaft or rod. Friction element can be attached to the shaft for frictionally engaging an inside surface of the housing. More than one friction element may be attached to the rod and each friction element can have an annular, xe2x80x9cdonut,xe2x80x9d shape. Each friction element can also have distinct inner and outer portions which are made of different compositions and have different properties. The means for generally maintaining the peripheral pressure between each friction element and the generally cylindrical housing within a preferred range of acceptable operating pressures can be integral with the housing such that the housing is self-adjusting. This may accomplished by configuring the housing in such a manner as to permit the housing to expand in a controlled manner in response to increased peripheral pressure. Thus, the expansion of the friction elements, which can occur due to the build up of heat during operation of the device, can be compensated for by the self-adjusting housing to generally maintain the peripheral pressure within a preferred range of pressure.
In another embodiment, the friction dampener can be very similar to the friction dampener described above, except having a generally rectangular housing in which a generally rectangular shaft is slidably enclosed. Similarly to the generally cylindrical shaped dampener, the friction elements are attached to the rectangular shaft for frictionally engaging inner walls of the rectangular housing. In this embodiment, a pair of rectangular friction elements can be attached to opposite sides of the rectangular shaft for engaging opposite inner surfaces of the rectangular housing. Each rectangular friction element can also have distinct inner and outer portions which can be made of different compositions and have different properties. A different means for generally maintaining the pressure between each friction element and the housing within a preferred range of acceptable operating pressures is also provided. In this case, the means can be accomplished by forming the housing in two opposed portions, one portion can be channel shaped and the other generally flat. By sizing the channel shaped portion appropriately, shims can be used between the two opposed housing portions to provide a certain preload on the friction elements when the two housing portions are fastened together with the shaft and friction element sandwiched therebetween. The preload is calculated to take into account anticipated thermal expansion of the friction elements such that the pressure between the friction elements and the housing does not exceed an acceptable range of operating pressures. Also, the width of the channel shaped member can also be sized slightly wider than the friction elements in order to provide some space between the edges of the friction material and the side walls of the housing to accommodate some amount of thermal expansion, thus reducing the buildup of pressure between the friction elements and the housing.
Other details, objects, and advantages of the invention will become apparent from the following description and the accompanying drawings of certain presently preferred embodiments thereof.